1. Field of the Invention
The present invention relates to a server-client type system and a data downloading method and, more particularly, to a server-client type system using ISDN lines and a data downloading method using ISDN lines.
2. Description of the Related Art
In a server-client type system in which a terminal adapter (TA) at a user home connected to a telephone, a facsimile device (hereinafter referred to as FAX), a personal computer (hereinafter referred to as PC), etc. is connected to a server through a first digital service unit (DSU), an Integrated Service Digital Network (ISDN) and a second DSU, at the time of downloading data existing in a client's terminal at the user home from the server, it is a conventional practice to start data downloading immediately at that time. On this occasion, the D-channel (Dch) packet exchange will serve for the downloading of a small volume of data (not more than several tens k bytes), while the same will take much time for the downloading of a large volume of data (several M bytes for program data etc. in general).
Under these circumstances, the data downloading method using Bch is conventionally known (Japanese Patent Laying-Open (Kokai) No. Heisei 4-120843). In the conventional data downloading method, a terminal device in advance downloads data to be downloaded into a line control device, while the line control device downloads the downloaded data into an apparatus accommodated in the ISDN network through Bch, thereby realizing data downloading through Bch having a high transmission rate independently of a transmission rate of a standard interface to reduce a downloading time.
For reference, time and charges for transmitting 4 M-byte data (4096 k bytes) within a relevant area are as follows. First, in a case of the Dch packet exchange, a transmission rate of Dch is 16 (k bytes/sec) and a transmission time of 4 M-byte transmission data will be accordingly 256 seconds (=4096 (k bytes)÷16 (k bytes/sec)). However, in a case of a packet, taking a header attached to each packet, and accumulation at an exchange and a transfer time into consideration, the transmission time will be approximately one and a half the above transmission time, that is, 384 seconds.
As to communication charges incurred in the Dch packet exchange, assuming that it costs 0.8 yen per packet and that transmission is conducted with one packet of 256 octets (0.25 k bytes), 4 M-byte transmission data is equivalent to 16384 packets (=4096 (k bytes)÷0.25 (k octets)) to charge 13107.2 (yen) (=16384 (packets)×0.8 (yen/packet).
In a case of the Bch packet exchange, a transmission rate of Bch is 64 (k bytes/sec), so that a transmission time of the 4 M-byte transmission data is 64 (sec) (=4096 (k bytes)÷64 (k bytes/sec)). However, since in a case of a packet, the transmission time will be approximately one and a half the above transmission time, that is, 96 seconds taking a header attached to each packet, and accumulation at an exchange and a transfer time into consideration.
As to communication charges incurred in the Bch packet exchange, assuming that it costs 6.2 yen per packet and that transmission is conducted with one packet of 4096 octets (4 k bytes), 4 M-byte transmission data is equivalent to 1024 packets (=4096 (k bytes)÷4 (k octets)) to charge 6348.8 yen (=1024 (packets)×6.2 (yen/packets)).
In a case of the Bch line exchange, a transmission time of the 4 M-byte transmission data is 64 (sec) (=4096 (k bytes)÷64 (k bytes/sec)) and its charge will be 10 yen within three minutes in the area. Calculation on the above transmission of data of 4 M-byte size will be summarized as follows.
TABLE 1THE KIND OF LINETIME(sec)CHARGES(yen)Dch packet exchange38413107Bch packet exchange966348Bch line exchange6410
As can been seen from the above Table 1, as to transmission of a large volume of data (of several M bytes), using the Bch line exchange taking charges and time into consideration is most preferable.
In the above-described conventional data downloading method, however, since the calculation results in the above Table 1 are obtained in a case where a transfer efficiency is 100 percent and there is no error, lines are occupied at least for 64 seconds, and at this time if other equipment at the user home is in use using the remaining Bch, which means that both of the two Bch lines are in use, even when the telephone should be used in emergency, it is necessary to wait for either line to be freed, which prevents efficient line use and sharing.